1. Field of the Invention
This invention relates to an Asynchronous Transfer Mode (ATM) data network switch for use in switching cells of data between a plurality of data links. The switch is arranged to have a high degree of tolerance to faults.
2. State of the Art
An ATM switch comprises, in general terms, a cross-point switch having a plurality of input ports and a plurality of output ports, and one or more controllers for switching data cells from any input port to any output port. The controllers which switch the data cells (which are often called "slot controllers" or "link controllers") each typically comprise input controllers or receivers, whose principal function is simply to receive the bit stream from the external link and to divide it up into cells for presentation to the switch fabric, and output controllers or transmitters, which serve to convert the separate cells from the switch fabric into a continuous bit stream again for forwarding on an appropriate external link.
Since a fault in the switch fabric could cause failure of the complete switch, duplicate switch fabrics connected in parallel to the slot controllers are used. If a fault is detected in one switch fabric, switching is transferred to the second switch fabric, while the first is removed from use. It is possible to designate one of the slot controllers as a system controller arranged to monitor operation of the switch. For example, the system controller can send out "health check" cells to each other controller, to which the other slot controllers are arranged to respond by returning the cell to the system controller, which monitors the responses received. If the system controller does not receive all responses, this may be due to a fault in the switch fabric, and the system controller then switches from the first to the second switch fabric. This can result in a cell loss.
A further problem with such an arrangement is that, although the switch fabric is fully duplicated, the second switch fabric remains inactive until it is required. It is therefore not possible to guarantee that the second switch fabric is fully operational when needed, since it can only be tested when in use. Further, no other advantage of duplication of switch fabrics is obtained. The capacity of the switch is identical with that of a switch having only a single switch fabric.